Ink printing apparatuses may be used for single-color or multicolor printing to a printing substrate, for example a single sheet or a web-shaped recording medium made of the most varied materials (paper, for example). The design of such ink printing apparatuses is known-see for example EP 0 788 882 B1. Ink printing apparatuses that operate according to the Drop-on-Demand (DoD) principle, for example, have as a printer one print head or multiple print heads with nozzle assemblies comprising ink channels and activators, wherein the activators—controlled by a printer controller—may excite ink droplets in the direction of the recording medium, which ink droplets are directed onto the recording medium in order to apply print dots there for a print image. The activators may generate ink droplets thermally (bubble jet) or piezoelectrically.
The design of a print head which has, for example, nozzle assemblies with piezoelectric activators is known from U.S. Pat. No. 7,281,778 B2. Each nozzle assembly comprises ink channels that end in nozzles arranged in a nozzle plate and provides activators that are respectively arranged at an ink channel. The recording medium is directed past the nozzle plate. If it should be printed, the activators provided for printing are activated by a printer controller that thereupon subjects the ink in the ink channels to pressure waves via which the ejection of ink droplets from the nozzles in the direction of the recording medium is induced.
Given low print utilizations of the ink printing apparatus, not all nozzles of the ink print heads are activated in the printing process. Many nozzles still have downtimes (printing pauses), with the result that the ink in the ink channel of these nozzles is not moved. Due to the effect of evaporation from the nozzle opening, the danger exists from this that the viscosity of the ink then changes. This has the result that the ink in the ink channel can no longer move optimally and, for example, can no longer exit from the nozzle. In extreme cases, the ink in the ink channel dries completely and clogs the ink channel, such that a printing with this nozzle is no longer possible.
The drying of the ink in the nozzles may be prevented in that printing occurs from all nozzles within a predetermined cycle. This cycle may be set corresponding to the print utilization. Individual points may thereby be applied in unprinted regions of the recording medium, or print dot lines may be printed between print pages. These methods may lead to disruptions in the print image, in addition to unnecessary ink consumption and additional wear of the print heads.
A drying of the ink in the nozzles of a print head in its printing pauses represents a problem that may also be prevented in that a purge medium (for example ink or cleaning fluid) is flushed through all nozzles in a purging process (also called purges) within a predetermined cycle. This purge cycle may be set corresponding to the print utilization (EP 2 418 087 A1).
The danger that nozzles of the nozzle units dry out additionally increases with decreasing humidity in the environment of the nozzle plate of the print head. This applies in particular to new ink types.
In U.S. Pat. No. 4,228,442 A, a drying of the ink in the nozzles of the print head is prevented in that the nozzle plate with the nozzles is arranged in a nozzle chamber to which a fluid is supplied that may then be evaporated in the nozzle chamber. The moisture in the nozzle chamber is thereby increased, and a drying of the ink in the nozzles is largely prevented.
US 2011/0273 510 A1, U.S. Pat. No. 8,622,538 B2 describe an ink printing apparatus with a print head unit with multiple print heads arranged in series. A recording medium is moved past the print heads with a transport arrangement. The transport arrangement has roller pairs between which the recording medium travels through. The roller pairs are respectively arranged before a print head. The unit made up of print heads and the recording medium are entirely encased in the region of the print head unit in order to avoid ink escaping from the print head unit into the environment. Provided before the print head unit is an arrangement that blows a damp gas into the housing in the direction of the print head unit, in parallel with the recording medium. Due to the roller arranged before the respective print head, this gas is swirled and thereby moves through below the adjacently situated print head. The exit of the print head is thereby dampened and a drying of the ink is prevented.
In JP 2005-271314 A, an ink printing apparatus is described in which ink vapor generated under the print heads during print operation should be removed. For this, a first arrangement is arranged to one side of the print heads as viewed in the transport direction of the recording medium, which first arrangement blows air in the gap between print head and recording medium, across a channel arranged at the one side wall of the print head. Arranged at the other side wall of the print head is an additional channel at which a second arrangement is provided that suctions the air with the ink vapor out of the gap. The air supplied via the first arrangement may be humidified.